Kofi Annan, the United Nations Secretary General, urged yesterday that an international force be sent to curb the violence in the Middle East, as the only way of preparing for negotiations.

Some form of outside peacekeeping force is known to be one idea under discussion by Colin Powell, the US Secretary of State, during his talks with Ariel Sharon and Yasser Arafat this weekend. Yesterday's suicide bombing in Jerusalem gave the proposal added urgency. The attack increased the frustration of President Bush and underlined again the inability of the US to impose its will on events.

The White House said that despite the attack, General Powell would meet Mr Arafat in Ramallah today as planned, but would deliver an even tougher message following what Mr Bush's spokesman described as a "homicide" bombing. "These aren't suicide bombings, they're murder," Ari Fleischer said.

The US now faces a huge dilemma. It can either – as seemed more likely yesterday – keep General Powell in the region, and risk seeing further embarrassment if mediation efforts come to nothing. Or it can bring him home, tacitly acknowledge America's powerlessness and further jeopardise relations with moderate Arab states.

Mr Annan, speaking in Geneva, warned that "the situation is so dangerous and the humanitarian and human rights situation so appalling, the proposition that a force should be sent in ... can no longer be deferred."

The killings on both sides were "an affront to the conscience of mankind". He said the force should be dispatched as soon as possible, but did not specify whether that should happen before a truce. Other UN officials said a deployment would depend on a ceasefire, an Israeli pullout and a series of confidence-building measures.

Mr Annan voiced concern at reports from UN agencies of "grave violations" by Israeli forces during the two-week offensive, and again urged Mr Sharon to withdraw. The violence has left diplomats scrambling for a formula to bring peace, and dramatic initiatives cannot be ruled out. A peacekeeping force is high on the list of options.

Another possibility is a joint initiative by the US, the UN, Europe and Russia, the four parties who appealed in vain in mid-week for an end to the offensive and a halt to the bombings. "There is maximum flexibility," Mr Fleischer said of the Powell mission.

However, major problems surround any peacekeeping force: its size and composition, and the minimum requirements for it to go in. Palestinians have always supported the idea, believing it would internationalise the crisis and help correct the imbalance of force. For exactly that reason, Israeli governments have always opposed it.

The Jewish state insists it has the right to self-defence. It also believes any genuinely multinational force would be sympathetic to the Palestinians. An all-US force would arouse misgivings in Washington, which has not forgotten how a similar exercise in Lebanon led to the death of 240 US servicemen in the 1983 Beirut barracks bombing.